Low-Delay CELP Vocoder
for TMS320C3x/'C4x
by DSP Software Engineering, Inc.

Software Overview

The LDCELP vocoder software is an implementation of DSPSE's 8-kbps proprietary low-delay CELP 
technology for the TMS320C3x. LDCELP is designed to encode 8-kHz sampled speech signals for 
transmission over 8-kbps channels. LDCELP provides approximately 4 kHz of speech bandwidth.
LDCELP encodes frames of 24 linear PCM samples into a 24-bit code word. LDCELP has numerous 
applications in systems that require high speech quality and low delay at a low digital bit rate 
such as videoconferencing, multimedia and digital telephony.

Features and Benefits

•	User-selectable processing frame size (any multiple of 24 samples)
•	Run-time selectable codebook size
•	4.5-ms algorithmic delay
•	In-band synchronization capable
•	Object-based interface
•	SPOX compatible
•	Independent, C-callable functions

Processor and System Requirements

•	Devices supported: TMS320C3x, TMS320C4x
•	Algorithm category: Vocoder
•	Requirements:
All memory requirements are in units of 32-bit words. The MIPs ratings presented require the listed 
amounts of on-chip, zero-wait-state program and data memory, and a processing frame size of 24 
samples. All measurements were made using an executable demonstration built for a third-party PC 
plug-in board.
Function
Avg MIPS
Chip RAM
Data
Program
Encoder (half duplex)
13.4
0.5 k
1.6 k
15.8 k
Decoder (half duplex)
3.3
0.2 k
1.8 k
15.8 k
Full Duplex*
16.7
0.7 k
2.0 k
15.8 k
* Significant portions of read-only program and data are shared by the encoder and decoder.

User Functions

The LDCELP implementation consists of two independent, C-callable functions that perform encoding 
and decoding operations. It also includes functions that reflect the implementation's object-based 
interface. The encoder and decoder interface with arrays of linear PCM samples and 24-bit code 
words.
LDCELP Encoder
LDCLPE_create (...)
Dynamically allocates and initializes data memory for encoder
LDCLPE_delete (...)
Frees encoder's usage of dynamically-allocated data memory
LDCLPE_encode (...)
Encodes a frame of linear PCM samples into a frame of code words
LDCELP Decoder
LDCLPD_create (...)
Dynamically allocates and initializes data memory for decoder
LDCLPD_delete (...)
Frees decoder's usage of dynamically allocated data memory
LDCLPD_decode (...)
Decodes a frame of code words into a frame of linear PCM samples

Availability

•	Source and object code currently available for licensing.

Company Background and Contact Information

DSP Software Engineering, Inc. (DSPSE), founded in 1989, is a leading provider of highly-complex 
digital-signal-processing software used in telecommunications and multimedia applications such as 
computer telephony, digital wireless, PSTN, and satellite communications, and videoconferencing.


Contact:	DSP Software Engineering, Inc.
		175 Middlesex Turnpike
		Bedford, MA 01730  USA
		(617) 275-3733
		Fax: (617) 275-4323
		e-mail:  info@dspse.com
		www:  http://www.dspnet.com